Couples contract civil marriage during a mass wedding ceremony on June 17, 2012 in Sao Paulo, Brazil. Sixty-two construction workers and their brides got married in the same ceremony at the new stadium of football team Corinthians, Itaquerao, which is being built for the FIFA World Cup Brazil 2014. AFP PHOTO / Nelson ALMEIDA (Photo credit should read NELSON ALMEIDA/AFP/GettyImages)

File photo of a married couple. (Photo credit NELSON ALMEIDA/AFP/GettyImages)

A long-term study conducted by researchers at the University of Wisconsin-Madison reveals that married couples who experience chronic marital stress are more vulnerable to symptoms of depression. And although past research suggests that that married people are generally happier than their single counterparts, this data finds that marriage is also one of the most prominent sources of long-lasting social stress.

Study leader Richard Davidson, the UW-Madison William James and Vilas Professor of Psychology and Psychiatry, says the findings to be published in the April 2014 edition of the journal Psychophysiology are very important in understanding mental health challenges within marriages.

“This is not an obvious consequence, if you will, of marital stress, but it’s one I think is extraordinarily important because of the cascade of changes that may be associated,” Davidson, founder of the Center for Investigating Healthy Minds at the UW’s Waisman Center, said in a statement through the university. “This is the signature of an emotional style that reveals vulnerability to depression.”

Consistent criticism, nagging and feeling let down by one’s partner were listed as some of the top triggers for dissatisfaction with a marriage. The long-term study observed couples after 9 and 11 years of marriage and asked questions related to how often they felt let down or how frequently their partner criticized them.

According to the UW-Madison news release, the participants’ reactions were measured in correlation to 90 mixed images of negative, neutral and positive photographs. The researchers viewed data relating to brain electrical activity, specifically the intensity and frequency revealed by the participants’ frowning muscles.

“It’s a nice way to get at what people are experiencing without asking people for their emotional response: ‘How are you feeling?'” says UW-Madison graduate student and study lead author, Regina Lapate. “How is it that a stressor gets under your skin and how does that make some more vulnerable to maladaptive responses?”

Couples who reported higher marital stress had shorter-lived responses to positive images than those who self-reported more satisfaction in their marriages. The researchers said that they were interested in analyzing how someone’s weakened ability to enjoy positive images or experiences could be linked to an inability to resist stress.

“To paraphrase the bumper sticker: ‘Stress happens,’” Davidson told PsychCentral.com. “There is no such thing as leading a life completely buffered from the slings and arrows of everyday life.”

A study published earlier this year by researchers at the Open Unviersity in the U.K. found that men and women without children are more satisfied in their relationships, and they are also more likely to feel valued by their partner.

“Saying ‘thank you’ and giving compliments emerged as one of the most important factor in keeping a relationship healthy across all groups,” the researchers noted in the study.